Method of making a carbon foam material and resultant product

ABSTRACT

A method of making anisotropic carbon foam material includes de-ashing and hydrogenating bituminous coal, separating asphaltenes from oils contained in the coke precursor, coking the material to create a carbon foam. In one embodiment of the invention, the carbon foam is subsequently graphitized. The pores within the foam material are preferably generally of equal size. The pore size and carbon foam material density may be controlled by (a) altering the percentage volatiles contained within the asphaltenes to be coked, (b) mixing the asphaltenes with different coking precursors which are isotropic in nature, or (c) modifying the pressure under which coking is effected. In another embodiment of the invention, solvent separation is employed on raw bituminous coal and an isotropic carbon foam is provided. A related carbon foam product is disclosed. The carbon foam materials of the present invention are characterized by having high compressive strength as compared with prior known carbon foam materials.

This application is a division of Ser. No. 08/887,556 filed Jul. 3,1997, now U.S. Pat. No. 5,888,469, which is a continuation of abandonedapplication Ser. No. 08/455,742 filed May 31, 1995 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of making an improved carbonfoam material and particularly a graphitized carbon foam material havingsuperior compressive strength and electrical conductivity.

2. Description of the Prior Art

It has been known for many decades that coal can be beneficiated forapplication in a wide variety of environments. For example, it has beenknown that coal may be employed as a fuel in electric utility plantsand, in respect of such usages, beneficiating of the coal will reducethe ash content and the amount of sulfur and nitrogen species containedin the gaseous exhaust products.

It has also been known to convert coal into coke for use in variousprocess metallurgy environments.

It has also been known to create carbon foam materials from feedstocksother than coal, which can be glassy or vitreous in nature, and arebrittle and not very strong. These products which lack compressivestrength tend to be very brittle and are not graphitizable. See,generally, Wang, “Reticulated Vitreous Carbon—A New Versatile ElectrodeMaterial,” Electrochimica Acta, Vol. 26, No. 12, pp. 1721-1726 (1981)and “Reticulated Vitreous Carbon An Exciting New Material,” UndatedLiterature of ERG Energy Research and Generation, Inc. of Oakland,Calif.

It has been known through the analysis of mechanical properties ofcarbon fibers that long-range crystallite orientation is achieved byalignment of the precursor molecules during fiber spinning. In“Idealized Ligament Formation in Geometry in Open-Cell Foams” by Hageret al., 21st Biennial Conference on Carbon, Conf. Proceedings, AmericanCarbon Society, Buffalo, N.Y., pp. 102-103 (1993), a model analysisregarding interconnected ligament networks to create geometricevaluation of hypothetical ligamentous graphitic foam is disclosed. Thismodel analysis, however, does not indicate that graphite foam was madeor how to make the same.

It has been suggested to convert petroleum-derived mesophase pitch intoa carbon foam product by employing a blowing/foaming agent to createbubbles in the material, followed by graphitization of the resultantcarbonized foams above 2300° C. See “Graphitic Carbon Foams: Processingand Characterizations” by Mehta et al., 21st Biennial Conference onCarbon, Conf. Proceedings, American Carbon Society, Buffalo, N.Y., pp.104-105 (1993). It is noted that one of the conclusions stated in thisarticle is that the mechanical properties of the graphitic cellularstructure were quite low when compared to model predictions.

It has been known to suggest the use of graphitic ligaments in anoriented structure in modeling related to structural materials. See“Graphitic Foams as Potential Structural Materials,” Hall et al., 21stBiennial Conference on Carbon, Conf. Proceedings, American CarbonSociety, Buffalo, N.Y., pp. 100-101 (1993). Graphitic anisotropic foams,when evaluated mathematically in terms of bending and bucklingproperties, were said to have superior properties when compared withother materials in terms of weight with particular emphasis on platestructures. No discussion of compressive properties is provided.

In “Carbon Aerogels and Xerogels” by Pekala et al., Mat. Res. Soc. Symp.Proc., Vol. 270, pp. 3-14 (1992), there are disclosed a number ofmethods of generating low-density carbon foams. Particular attention isdirected toward producing carbon foams which have both low-density (lessthan 0.1 g/cc) and small cell size (less than 25 microns). This documentfocuses upon Sol-gel polymerization which produces organic-basedaerogels that can be pyrolyzed into carbon aerogels.

In “Carbon Fiber Applications,” by Donnet et al., “Carbon Fibers,”Marcel Decker, Inc., pp. 222-261 (1984), mechanical and other physicalproperties of carbon fibers were evaluated. The benefits and detrimentsof anisotropic carbon fibers are discussed. On the negative side, arethe brittleness, low-impact resistance and low-break extension, as wellas a very small coefficient of linear expansion. This publication alsodiscloses the use of carbon fibers in fabric form in order to providethe desired properties in more than one direction. The use of carbonfibers in various matrix materials is also discussed. A wide variety ofend use environments, including aerospace, automotive, road and marinetransport, sporting goods, aircraft brakes, as well as use in thechemical and nuclear industries and medical uses, such as in prostheses,are disclosed.

It has been known to make carbon fibers by a spinning process atelevated temperatures using precursor materials which may bepolyacrylonitrile or mesophase pitch. This mesophase pitch is said to beachieved through conversion of coal-tar or petroleum pitch feedstockinto the mesophase state through thermal treatment. This thermaltreatment is followed by extrusion in a melt spinning process to form afiber. The oriented fiber is then thermoset and carbonized. To make ausable product from the resulting fibers, they must be woven into anetwork, impregnated, coked and graphitized. This involves a multi-step,costly process. See “Melt Spinning Pitch-Based Carbon Fibers” by Edie etal., Carbon, Vol. 27, No. 5, pp. 647-655, Pergamen Press (1989).

There remains, therefore, a very substantial need for an improved methodof making carbon foam product which has enhanced compressive strengthand is graphitizable and the resultant products.

SUMMARY OF THE INVENTION

The present invention has met the above-described needs. In onepreferred method of the present invention, a coke precursor is providedby de-ashing and hydrogenating bituminous coal. The hydrogenated coal isthen dissolved in a suitable solvent which facilitates de-ashing of thecoal and separation of the asphaltenes from the oil constituent. Theasphaltenes are subjected to coking, preferably at a temperature ofabout 325° C. to 500° C. for about 10 minutes to 8 hours to devolatilizethe precursor asphaltenes. The coking process is preferably effected ata pressure of about 15 to 15,000 psig. The anisotropic carbon foam socreated is then cooled. In a preferred practice of the invention, theanisotropic carbon foam so created is subsequently graphitized. As analternate to employing hydrogenation, solvent de-ashing of the raw coalalone may be employed in order to create the asphaltenes which are thencoked and graphitized in the same manner. With this approach, anisotropic product is produced from the solvent extraction of rawunhydrogenated coal.

In a preferred practice of the invention, a blend of hydrogenated andunhydrogenated solvent separated asphaltenes may be employed in order toadjust the degree of anisotropy present in the carbon foam. Also, it ispreferred that the voids in the foam may be generally of equal size. Thesize of the individual bubbles or voids may be adjusted by altering theamount of volatile material contained in the asphaltenes or varying thepressure under which coking is effected.

In a preferred practice of the invention, after coking, the foamedmaterial is subjected to calcining at a temperature substantially higherthan the coking temperature to remove residual volatile material. Thepreferred temperature is about 975° C. to 1025° C. and the time is thatwhich is adequate to achieve a uniform body temperature for thematerial.

The method produces a graphitized carbon foam product having acompressive strength in excess of about 600 lb/in².

It is an object of the present invention to provide a method ofproducing coal-derived carbon foam which may be graphitized.

It is a further object of the present invention to provide such a methodand the resulting product which may be produced by hydrogenatingbituminous coal followed by separation of asphaltenes, and coking thesame.

It is a further object of the present invention to provide a method andresultant product which permits control of the degree of anisotropy ofthe carbon foam.

It is a further object of the present invention to provide such a methodwherein solvent partitioning of the unhydrogenated coal or hydrogenatedcoal is employed to select the proper fraction for making the desiredfoam or removing inorganic species from the coal.

It is a further object of the present invention to provide such a methodwhich permits control over the size of the voids in the carbon foam andthe density of the same.

It is a further object of the invention to provide a method of producingsuch a product which is capable of being graphitized and has much highercompressive strength than previously known carbon foams.

It is a further object of the present invention to provide such a methodwhich produces a controllable, low-density carbon foam product havingeither isotropic or anisotropic graphite structure which may haveopen-cell or close-cell configurations.

It is a further object of the present invention to provide a method ofproducing such a product which is lightweight and possesses acontrollable degree of electrical and thermal conductivity.

These and other objects of the invention will be more fully understoodfrom the following detailed description of the invention on reference tothe illustrations appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a first embodiment of the method ofproducing anisotropic graphitized carbon foam of the present invention.

FIG. 2 is a second embodiment of a method of producing isotropicgraphitized carbon foam of the present invention.

FIG. 3 is a schematic diagram of a preferred method of controlling thedegree of anisotropy.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred practice of the present invention, bituminous coal isprovided in size of about −60 to −200 mesh and, preferably, about −60 to−80 mesh. In the embodiment, as shown in FIG. 1, wherein anisotropiccarbon foam will be produced, the coal is first hydrogenated in step 2.This may be accomplished at a temperature of about 325° C. to 450° C. ata hydrogen over-pressure of about 500 to 2,500 psig hydrogen cold for areaction time of about 15 minutes to 1.5 hours. Tetralin may be employedas a proton-donating agent. After the reactor has cooled, the contentsare removed and the tetralin separated by distillation. The resultinghydrogenated coal may be exhaustively extracted with Tetrahydrofuran(THF) with the residue being filtered to remove inorganic matter. It hasbeen found that under these hydrogenating conditions, more than one-halfof the coal mass will be rendered soluble in THF. The hydrogenated coalis subjected to preliminary beneficiation as by de-ashing, which may beaccomplished in the manner disclosed in Stiller et al., U.S. Pat. No.4,272,356. The THF portion will contain all of the asphaltenes, orcoal-derived pitch precursors, and oils. The THF, after extraction iscomplete, may be evaporated for recycling and the recovered coal-derivedpitch precursor separated by employing a suitable solvent, such astoluene. The toluene-soluble fraction will be referred to generally as“oils” and the remainder referred to as the asphaltene fraction orcoal-derived pitch precursor fraction which is dried. This solventseparation step is identified as step 10 in FIG. 1.

The next step 12 in creating anisotropic carbon foam is to coke theasphaltenes. Coking is preferably achieved at a temperature of about325° C. to 500° C. for a period of about 10 minutes to 8 hours and,preferably, about 15 minutes to 5 hours in an inert gas atmosphere, suchas nitrogen or argon gas in a range of about 15 to 15,000 psig (hot) andpreferably about 50 to 1000 psig (hot). The pressure is maintained at agenerally constant level by means of ballast tank 16.

In a preferred practice of the invention, after coking, the foamedmaterial is subjected to calcining at a temperature substantially higherthan the coking temperature to remove residual volatile material. Thepreferred temperature is about 975° C. to 1025° C. and the time is thatwhich is adequate to achieve a uniform body temperature for thematerial.

In a preferred practice of the invention, heating of the oven begins at350° C. and the temperature is raised at a rate of 2° C. per minuteuntil the temperature reaches 450° C. at which temperature is held forabout 5 to 8 hours. After heating, the oven is turned off and thecontents are allowed to cool to room temperature slowly, generally overa 5 to 8 hour period. This foaming operation, when conducted in thecoking oven in this manner, serves to partially devolatilize theasphaltenes with the evolution of the volatile matter serving to providebubbles or voids and thereby create the carbon foam product. If desired,the carbon foam may be used in this form for many uses, such asstructural materials, light-weight automotive composites, impact andenergy-absorbing structures and heat insulators, for example.

In a most preferred practice of the invention, the carbon foam issubjected to graphitizing (step 14) which is preferably accomplished ata temperature of at least 2600° C. The time of the graphitizing step 14must be continued for a time period long enough to achieve a uniformtemperature of at least 2600° C. throughout the entire foam material.The larger the specimen, the longer the time required. A small specimenmay require about one hour, for example. In the most preferred practice,the process of graphitizing is accomplished at about 2600° C. to 3200°C.

In the embodiment of FIG. 1, hydrogenation involves large coal moleculesbeing broken apart thermally with the resultant fragments being cappedby hydrogen. This results in formation of smaller aromatic molecules.The hydrogenated coal is subjected to solvent extraction by boiling witha solvent to solubilize most of the organic material in the coal. Thisremoves the inorganic impurities which are not soluble and, therefore,are removed by simple filtration. A second solvent may be employed toseparate the desired asphaltene fraction. Once the solvent isevaporated, the resulting extract is a solid which is free from allinorganic impurities.

In order to provide further disclosure regarding this first embodiment,an example will be considered.

EXAMPLE 1

Raw Pittsburgh #8 Coal was hydrogenated by introduction into anautoclave reactor at 350° C. for one hour under 1000 psig (cold)hydrogen using 3:1 (weight) tetralin to coal ratio. After cooling, thehydrogenated coal was removed from the reactor and the tetralin wasevaporated. The hydrogenated coal was extracted in THF and the residuewas filtered to remove the inorganic species. The THF was evaporatedfrom the filtrate and the resultant extract was dissolved in toluenewith the undissolved portion being filtered. The toluene was evaporatedto recover the extract. The THF soluble/toluene insoluble (asphalteneportion) extract was placed in a reactor with the nitrogen pressure setat 700 psig in order to effect the foaming operation. The startingtemperature was 350° C. and the temperature was elevated at 2° C./minuteuntil 450° C. was reached. It was held at the temperature for five hoursand then calcined at 1000° C., after which it was cooled slowly. Thefoam was then removed from the reactor. The foam was then graphitized byintroducing it into a furnace at a temperature of 2600° C. and apressure of 0 psig argon wherein it was held for one hour which was thetime required to heat the particular specimen to a uniform temperatureof 2600° C. and, after cooling, was withdrawn. This produced ananisotropic graphitized carbon foam.

In an alternate embodiment of the invention, the method may be practicedessentially as is shown in FIG. 1, except for elimination of thehydrogenation step 2. This approach is illustrated in FIG. 2 wherein thecoal is de-ashed (step 22) in a solvent, such as N-methyl pyrrolidone,for example. There is asphaltene separation (step 30), followed bycoking of the asphaltenes to create carbon foam (step 32), under theinfluence of ballast tank 33, and subsequent graphitizing of the carbonfoam (step 34). In effecting the solvent extraction, it is preferredthat the solvent-to-bituminous coal ratio be about 3:1 to 10:1 on aweight basis and heating to the solvent boiling point.

Calcining may be effected after foaming and before graphitizing. Thiscalcining may be effected at about 975° C. to 1025° C.

The alternate approach of FIG. 2 produces an isotropic foam carbon whichmay be graphitized. The bubble or pore sizes of this embodiment will allbe equal and control of size may be effected by controlling the amountof volatiles in the asphaltenes, as well as the externally appliedpressure through ballast tank 33.

Referring to FIG. 3, the degree of anisotropy may be altered by blending(step 44) the hydrogenated asphaltenes (step 40) from step 10 in FIG. 1with unhydrogenated asphaltenes in a solvent (step 42) from step 30 ofFIG. 2, the latter of which tends to be isotropic. This produces thedesired anisotropic characteristics in the final product. The desireddegree of anisotropy may be adjusted in this manner in order to providethe preferred properties for the particular end use contemplated. Thesolvent is evaporated in step 46 after which the asphaltenes are cokedin step 48 to create carbon foam and then the carbon foams aregraphitized in step 50. Ballast tank 49 is employed to maintain thepressure at the desired level.

With respect to the bubble or void dimension in the foam, the bubbleswill generally be of equal dimension to each other. The bubble dimensionmay be varied by altering the volatile content of the asphaltenesobtained through the extraction process.

Also, the bubble dimension and density of the carbon foam can be alteredby altering the external pressure applied through a ballast tank 16which is operatively associated with the coking oven (step 12) (FIG. 1),as well as ballast tanks 33, 49 (FIGS. 2 and 3). The higher the ballastpressure applied externally, the smaller the bubble dimension and thehigher the density of the resultant product.

The graphitized foams made by the present invention will have a bulkdensity of about 0.2 to 2 g/cc and, preferably, be about 0.2 to 0.4g/cc.

It has been found that the graphitized foam product produced in thismanner has a very high compressive strength and generally will begreater than about 600 lb/in². The compressive strength is related tobubble size with smaller bubble size increasing compressive strength.

If desired, pitch material may be introduced into the voids whichcommunicate with the exterior of the carbon foam so as to enhance thestrength. This pitch may be a standard petroleum-based impregnationpitch. This product may be coked in order to devolatilize and strengthenthe inserted pitch material. The pitch filling the voids may be bakedand graphitized.

Among the advantageous properties of the present carbon foam are that itis lightweight and can be either a thermal insulator, which iselectrically conductive, or an efficient conductor of heat andelectricity. The foam can be molded into any desired shape by cokingwithin appropriately shaped molds. This provides numerous potential enduses. The material may be used, for example, in membranes for separationof ions in solution or particulates in gases at high temperature, it canbe used in thermal management devices, such as substrates for integratedcircuits or aerospace applications. The material may also be employed infilters including high temperature filters. Also, depending upon the enduse, the degree of thermal conductivity can be altered. In general, theisotropic foam is not a high thermal conductor and the non-graphitizedfoam is a poor thermal conductor. The foam may also find use in buildingand structural members, such as a substitute for wood and steel beams.Numerous, advantageous automotive uses, such as in pistons, vehicleframes, impact absorbers for doors, and connecting rods exist. Further,in view of the strength and lightweight nature of the product, aerospaceand airplane uses, such as in wings, brakes, as well as satellite andspace station structures involve advantage end uses.

The carbon foam product resulting from the captioned invention may beanisotropic as in the first embodiment (FIG. 1) or isotropic (FIG. 2),or with the degree of anisotropic nature being controlled by the natureof the asphaltene materials mixed (FIG. 3). One means of creating thedesired level of anisotropic properties would be to admix apredetermined ratio of a solvent extract from hydrogenated coal with theextract from unhydrogenated coal as in FIG. 3, in a solvent, such asN-methyl pyrrolidone (NMP). The resultant coal-derived pitch precursoror asphaltene produced after solvent evaporation is employed in thecoking operation. By adjusting the ratio of unhydrogenated tohydrogenated coal-derived pitch precursor products, different levels ofanisotropy may be achieved. In general, the carbon foam materials willhave a compressive strength greater than 600 lb/in². In addition toimpregnating the foam material with pitch to fill the voids, if desired,such pitch filled void materials may be baked and graphitized, ifdesired. An alternate approach, for some uses, would be to cover thefoam with a thin skin of carbon fibers or pitch to seal the outersurface. This would result in a lightweight, yet strong, structuralmember. In uses such as a gas or liquid filter for environmentalremediation, for example, it may be desirable to activate the surfacearea of the material.

While it will be appreciated by those skilled in the art that numeroususes may be made of a material having the blend of desired properties ofthose of the present invention, among additional uses would be as acatalyst support for high temperature catalysts, in biologicalmaterials, such as bone and prosthetic items, as well as inenvironmental waste remediation as by heavy metal removal, electrostaticprecipitators and in nuclear waste containers wherein undesired leachingor degradation of the carbon material would be resisted.

It will be appreciated that the present invention has provided a methodof creating a unique, high-compressive strength, coal-derived carbonfoam material, which may be graphitized, and also possesses a greatnumber of desirable properties. All of this is accomplished by startingwith bituminous coal and employing either the hydrogenated anisotropicapproach or the solvent approach without prior hydrogenation to producean isotropic structure or a blend of the two materials to produce thedesired level of anisotropy. In addition, the void or bubble size anddensity of the carbon foam may be controlled by adjusting the volatilecontent of the coal-derived pitch precursor or asphaltene which is to becoked and/or adjusting the ballast pressure which is applied externally.This void or bulk size is controlled without requiring dependence on anexternal blowing/foaming agents. The solvent separation of step 10 ofFIG. 1 and step 30 of FIG. 2 serve to remove inorganic species and/oralso to select the proper fraction to make the desired foam.

The process of producing a foam depends primarily on two features: (a)the presence of volatile material which, in the preferred practice ofthe invention, will be internally present; and (b) the viscosity of theparent pitch may be altered by cross-linking which results fromdevolatilization or the presence of an externally-added cross-linkingagent. If desired, for example, a plasticizer may be employed to adjustthe viscosity to the desired value. At high viscosity, pitch flow isrestricted and this resists the coalescence of bubbles produced bydevolatilization. This results in a high-density foam with small bubblesize. At lower viscosity, bubbles can coalesce and a more open foamstructure with larger bubbles is produced. The devolatilization iscontrolled by application of external pressure from a ballast tank, suchas 16, 33, or 49. At high pressure, less movement of the bubbles takesplace and, as a result, the foams are more dense. At low externalpressure, the bubbles tend to coalesce and flow so a less-dense foam isproduced.

Whereas particular embodiments of the invention have been describedherein for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details may be made withoutdeparting from the invention as set forth in the appended claims.

We claim:
 1. A method of making an anisotropic carbon foam materialcomprising hydrogenating and de-ashing bituminous coal, converting saidhydrogenated bituminous coal into asphaltenes and oils in a solvent,separating said asphaltenes from said oils, coking said asphaltenes byheating at a temperature of about 325° C. to 500° C. for about 10minutes to 8 hours to devolatize and foam said asphaltenes at a pressureof about 15 to 15,000 psig and cooling said carbon foam, graphitizingsaid carbon foam at a temperature of at least about 2600° C., after saidcoking but prior to said graphitizing, calcining said carbon foam,creating said carbon foam with voids of generally uniform size, wherebysaid bituminous coal is converted into an anisotropic, calcined,graphitized, carbon foam having voids of generally uniform size, andcontrolling said void size to create the desired compressive strength ofsaid carbon foam.